There are four main types of leukemia. They are grouped according to how quickly the disease progresses and which white blood cells are affected.

Acute leukemia progresses rapidly. Chronic leukemia progresses more slowly. There may be an increased number of the white cells that play an important part in the immune system (lymphocytes) or an increased number of the white cells that normally help the body fight infection (myelocytes). These cells are abnormal and do not work properly. The four main types of leukemia are:

• Acute lymphoblastic leukemia (ALL). ALL is the most common type of leukemia in children but may also affect adults.
• Acute myelogenous leukemia (AML). AML is the most common type of leukemia among both children and adults. A subtype of AML is acute promyelocytic leukemia (APL).
• Chronic lymphocytic leukemia (CLL). CLL mostly affects people older than 55 years of age and almost never affects children.
• Chronic myelogenous leukemia (CML). CML occurs mostly in adults.